I am running Flex 3 on Windows Vista. For some reason, attempting to profile an application would always fail. There were no errors or warnings… the app would launch in my browser and simply run without profiling.

The following work around worked for me:

In Flex 3, click on Window > Preferences…

In the Preferences window, choose Flex > Profiler > Connections

Set the Port number to something other than 9999. 10001 worked for me.